A supercapacitor or electrochemical capacitor is a capacitor of particular technology that makes it possible to obtain power and energy densities that are intermediate between batteries and conventional electrolytic capacitors. Most supercapacitors comprise two porous electrodes impregnated with electrolyte and separated by an insulating and porous membrane that allows the circulation of the ions contained in the electrolyte.
The basic principle of supercapacitors rests upon the capacitive properties of the interface between the electrodes which are solid electronic conductors and the electrolyte which is a liquid ionic conductor. Energy storage takes place by the distribution of the ions of the electrolyte in the vicinity of the surface of each electrode, under the influence of the potential difference applied between the two electrodes. A space charge region, also known as an electrochemical double layer, having a thickness limited to a few nanometers is thus created at the interfaces. The supercapacitors therefore have full capacitances. The energy storage is in fact of electrostatic origin and not of electrochemical origin as in the case of storage batteries, which gives them a potentially high specific power.
Conventionally, supercapacitors use activated carbon as electrode material. Research is being carried out to develop electrodes based on CNTs, the acronym for carbon nanotubes. Carbon nanotubes have an electrical conductivity of the order of 100 S·cm−1, greater than the electrical conductivity of activated carbon which is of the order of 1 S·cm−1. Carbon nanotubes are easy to process; they do not require the use of binder, for example. Furthermore, the use of sheets of carbon nanotubes, more commonly known under the name “buckypaper”, makes it possible to envisage the production of flexible supercapacitors owing to the mechanical cohesion of the carbon nanotubes to one another in a buckypaper.
Customarily, the electrolytes used are based on the use of a quaternary ammonium salt, such as tetraethylammonium tetrafluoroborate, dissolved in an organic solvent, typically acetonitrile or propylene carbonate, which potentially has significant risks of leakage of the solvent in the case of damaging the supercapacitor.
An improvement that makes it possible to prevent the risks of leakage consists in producing all-solid supercapacitors, or, in other words, supercapacitors comprising a solid electrolyte, i.e. supercapacitors comprising an electrolyte which does not flow.
Several routes are being explored in order to obtain a solid electrolyte:
One route consists in producing a polymer electrolyte. A polymer electrolyte is understood to mean: a polymer that can dissolve a salt or an ionic liquid or a polymer synthesized in an ionic liquid.
This first route makes it possible to combine properties of ionic conductivity with a polymer.
Here are several examples of polymer electrolytes obtained by dissolving a salt or an ionic liquid in a polymer:
mixture of polyacrylonitrile or polyethyleneoxide or polyvinyl alcohol with an ionic liquid, “A. Lewandowski, A. Swiderska, Solid State Ionics, 169 21-24, (2004)”,
mixture of polyvinyl alcohol with a phosphomolybdic acid, “Y. Zhang, X. Sun, L. Pan, H. Li, Z. Sun, C. Sun, B. K. Tay, J. of Alloys and Compounds, 480, L17-L19, (2009)”,
mixture of polyvinyl alcohol or sulfonated polyether ether ketone with a lithium salt, “M. S. Kumar, D. K. Bhat, J. of Applied Polymer Science, 114, 2445-2454, (2009)”.
Here are several examples of polymer electrolytes obtained by radical polymerization of a monofunctional monomer in solution in an ionic liquid:
methyl methacrylate or acrylonitrile or vinyl acetate or styrene or 2-hydroxyethyl methacrylate (HEMA) polymerized in solution in 1-ethyl-3-methylimidazolium tetrafluoroborate or in 1-butylpyridinium, “A. Noda, M. Watanabe, Electrochimica Acta 45. 1265-1270, (2000)”,
methyl methacrylate or acrylonitrile or vinyl acetate or styrene or 2-hydroxyethyl methacrylate (HEMA) or methyl acrylate or acrylamide polymerized in solution in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI), “M. A. B. H. Susan, T. Kaneko, A. Noda, and M. Watanabe, J. AM. CHEM. SOC., 127, 4976-4983, (2005)”.
Another route consists in using polymers bearing ionic functions, mainly of ionic liquid type such as the polymers of FIGS. 1a to 1d, for which the charge is borne by the side chains of the macromolecule:
(a) poly(1-vinyl-3-ethylimidazolium) salt,
(b) poly(1-(6-(acryloyloxy)hexyl)-3-ethylimidazolium salt,
(c) ionic liquid polymer with a side chain, derived from a methacrylate and bearing a 3-ethylimidazolium group as cation,
(d) ionic liquid polymer with a side chain, functionalized by a trifluoromethane sulfonimide anion and having 1,3-ethylmethylimidazolium as counter ion.
FIG. 2 represents a polymer, obtained by self-assembly of a diborylated ionic liquid with 1,4-diazabicyclo[2.2.2]octane, for which the charge is borne by the backbone of the macromolecule.
In this second route, the ionic conductive properties are intrinsic to the polymer and are provided by the ionic functions borne by the polymer.
The routes developed above make it possible to produce solid electrolytes, however the ionic conductivities obtained do not attain the desired performance in terms of power.
A third route requiring networks of polymers formed in the presence of a liquid electrolyte is envisaged.
The polymer network traps a large amount of liquid electrolyte thus forming a two-phase network that may resemble a solid.
This development route makes it possible to avoid problems of leakage. Furthermore, the weight fraction of polymer network is low, the electrolyte predominantly consisting of a liquid phase. The large amount of liquid phase enables a significant increase in the ionic conductivity relative to the ionic conductivities obtained by the routes described above.
The gels obtained by creation of weak bonds between the polymer and the liquid phase are known as physical gels. Two procedures exist for synthesizing these physical gels.
A first procedure consists in dissolving a triblock polymer of ABA type in a solvent that is selective for block B, for example. Here are several examples of solutions that can form physical gels according to this method:
poly(styrene-block-ethylene oxide-block-styrene) (SOS) dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][BF6]), ([BMIM][PF6]) being an ionic liquid that is a solvent for polyethylene oxide, “Yiyong He, Paul G. Boswell, Philippe Bohlmann, and Timothy P. Lodge, J. Phys. Chem. B, 111, 4645-4652, (2007)”,
poly(styrene-block-methyl methacrylate-block-styrene) triblock copolymer dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMI][TFSI], “Keun Hyung Lee, Sipei Zhang, Timothy P. Lodge, and C. Daniel Frisbie, J. Phys. Chem. B, 115, 3315-3321, (2011)” and “Keun Hyung Lee, Moon Sung Kang, Sipei Zhang, Yuanyan Gu, Timothy P. Lodge, and C. Daniel Frisbie, Adv. Mater., 24, 4457-4462, (2012)”.
A second method of preparing the gels consists of the evaporation of a mixture comprising a solvent and a non-solvent in which two polymers having different solubilities are dissolved. The evaporation of the non-solvent creates micropores which, in a second step, are filled with a solution of electrolyte in an organic solvent. Here is an example of a solution that may form a physical gel according to this other method:
polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA) as polymers, dimethylformamide (DMF) as solvent and glycerol as non-solvent, LiPF6 in a mixture of dimethyl carbonate and ethylene carbonate as liquid electrolyte.
Furthermore, by introducing finely divided silica into a solution of sulfuric acid a physical gel is obtained owing to the hydrogen bonds that are created between the silica grains and the sulfuric acid solution, “J. M. Ko, R Y. Song, H. J. Yu, J. W. Yoon, B. G. Min, D. W. Kim, Electrochimica Acta, 50, 873-876, (2004).
The so-called “physical” gels are in essence reversible and, consequently, not very stable. Several research routes have made it possible to develop irreversible gels by crosslinking of the polymer network. Here are several examples:
reaction of a polyethylene glycol functionalized by amine groups with a polyethylene glycol terminated by succinimide functions in the presence of an ionic liquid, “Marc A. Klingshirn, Scott K. Spear, Raman Subramanian, John D. Holbrey, Jonathan G. Huddleston, and Robin D. Rogers, Chem. Mater. 16, 3091-3097, (2004)”,
crosslinking of an epoxy resin in the presence of an ionic liquid, “Kozo Matsumoto and Takeshi Endo, Macromolecules, 41, 6981-6986, (2008)”,
crosslinking of a mixture of acrylamide (monofunctional monomer) with N,N′-methylenebisacrylamide (bifunctional monomer) and acrylamide-2-methylpropanesulfonic acid (monofunctional monomer) with aqueous hydrogen peroxide solution in the presence of LiClO4, “B. Ganesh, D. Kalpana, N. G. Renganathan, Ionics, 14, 339-343, (2008)”,
electron beam crosslinking of a mixture of polyoxyethylene and of polyethylene glycol diacrylate (PEGDA) in the presence of an ionic liquid (LITFSI), “R. Uchiyama, K. Kusagawa, K. Hanai, N. Imanishi, A. Hirano, Y. Takeda, Solid State Ionics 180, 205-211, (2009)”,
photochemical crosslinking of 1,6-hexanediol diacrylate (HDDA) in the presence of an ionic liquid dissolved in an oligomer of polyethylene glycol dimethyl ether (PEGDME), “Da Qin, Yiduo Zhang, Shuqing Huang, Yanhong Luo, Dongmei Li, Qingbo Meng, Electrochimica Acta 56, 8680-8687, (2011)”,
crosslinking of a butadiene-acrylonitrile copolymer terminated by amine functions (ATBN) with a polyhedral oligomeric silsesquioxane (POSS) functionalized by epoxycyclohexyl groups, “Ming Li, Wentan Ren, Yong Zhang, Yinxi Zhang, Journal of Applied Polymer Science, 126, 273-279, (2012)”,
Michael addition reaction between a molecule bearing four thiol functions and a mixture of difunctional and trifunctional acrylates, “Devatha P. Nair, Neil B. Cramer, John C. Gaipa, Matthew K. McBride, Emily M. Matherly, Robert R. McLeod, Robin Shandas, and Christopher N. Bowman, Adv. Funct. Mater., 22, 1502-1510, (2012)”.
The routes described for obtaining gels by crosslinking organic compounds only permits a limited choice of the organic compounds that can be used. Furthermore, the use of large-sized functionalized polymers prohibits their impregnation in the pores of the carbonaceous materials.
Indeed, as FIG. 3 indicates, the surface of a carbonaceous material 1 comprises macropores 2 having a size of greater than 50 nm, mesopores 3 having a size of between 2 and 50 nm and micropores 4 having sizes of less than 2 nm. It is easily understood that monomers of excessively large size cannot fit into all the pores of the carbonaceous material 1.
The exchanges between the carbonaceous material and the gel formed from large-sized macromolecules are not very good which reduces the specific capacitance of the supercapacitors that use electrodes based on carbonaceous material.
Another route consists in polymerizing a mixture of monofunctional and polyfunctional monomers dissolved in an ionically conductive liquid by thermally-initiated radical polymerization, the polymerization being carried out in situ in the presence of the electrode material. The ionically conductive liquid is possibly an ionic liquid, or an aqueous or organic solvent.
This route has the following advantages:
it makes it possible to obtain a solid electrolyte which does not flow,
it enables the use of small-sized precursor monomers enabling their impregnation within the micropores 4 of the carbonaceous material,
it enables a better control of the impregnation time, the crosslinking being initiated by heating.
These advantages make it possible to create a good contact between the carbonaceous material and the gel thus making it possible to obtain a specific capacitance of the supercapacitor of the same order of magnitude as that obtained with a liquid electrolyte.
However, it should be noted that the carbonaceous materials prevent the crosslinking of the monomers.
Indeed, certain carbonaceous materials have sites which are free radical traps or, in other words, the carbonaceous materials have sites for capturing free radicals.
The free radicals formed during the initiation of the monomer crosslinking reaction are then fixed or “trapped” on these sites. This fixation of the radicals inhibits the in situ crosslinking reaction of the monomers. It then becomes impossible to form a gel via a thermally-initiated radical route in the presence of a material comprising porous activated carbon.